DESCRIPTION: The experiments described in this proposal are designed to increase understanding of the mechanisms by which DNA is repaired and recombined in eukaryotic cells. The specific goals are concerned with pathways of the DNA mismatch repair system, which is responsible for the correction of non-complementary bases in the DNA helix. The proposed studies utilize S. cerevisiae as a model organism as it has proven to be spectacularly successful in yielding information about the mismatch repair system in all eukaryotes, including humans. The importance of the mismatch repair system to human health has been demonstrated by the finding that the majority of hereditary nonpolyposis colon cancer (HNPCC) is due to germline mutations in one of several mismatch repair genes. In addition, mutations in mismatch repair genes have been observed in many other types of cancer. In knockout mice, several mismatch repair genes have been demonstrated to be tumor suppressor genes. Perhaps equally important, mutations in mismatch repair genes lead to resistance to commonly used chemotheraputic drugs. Thus the mismatch repair system plays a role in both the cause and treatment of cancer. Because so much is still not understood about the mismatch repair system, it is likely that additional effects of mutations in the system will be found. These studies focus on the yeast genes that are involved in nuclear mismatch repair: MSH2, MSH3 and MSH6. The products of these genes are responsible for the recognition of base distortions in the DNA and therefore begin the process of repair. The specific aims of this proposal are as follows. 1) To determine the specificity of mismatch repair and replication proofreading by using a base-specific reversion assay. 2) To determine the effect of the proofreading and mismatch repair systems on certain types of damaged bases in yeast. 3) To determine the effect of mismatch repair on blocking recombination between nonidentical sequences in yeast by using a competitive recombination assay. 4) To determine aspects of the structure and function of the yeast mismatch repair genes by examining phenotypes associated with specific mutations.